Most electronic devices are powered by an AC-DC power supply that receives alternating current (AC) power from an AC line at a nominal voltage in the range from 100 volts to 240 volts RMS and a frequency of 50 Hz or 60 Hz and supplies direct current (DC) power at one or more voltages in a range from about 1 volt to 50 volts. Conventionally, a magnetically-coupled transformer whose primary is connected to the AC line is used to step the AC line voltage down to the voltage needed by the electronic device. The output of the transformer is rectified, filtered with a large capacitor to remove the ripple component and regulated to provide the DC power for the electronic device. Problems with this solution include the large size and weight of the transformer, the large physical size of the ˜0.1 Farad capacitor used to filter the ripple component, and the inherent one-off type process used to fabricate the power supply. The large size and weight of the transformer and the large size of the capacitor are the result of the low frequency of the AC line.
The physical size and weight of the power supply can be reduced by rectifying the AC line voltage, filtering the resulting raw DC, chopping the filtered DC at a frequency typically of the order of tens of kilohertz, and using a transformer to step the voltage of the chopped DC down to the voltage needed by the electronic device. The output of the transformer is then rectified, filtered and regulated. With this solution, the transformer and the second filter capacitor are smaller than the transformer and filter capacitor in the previous example because they operate at a much higher frequency. However, the first filter capacitor is still large because it operates at the AC line frequency. Moreover, a one-off type process is needed to fabricate at least part of such switching-mode power supply.